This invention relates to surface-modified LiCoO2 powders, applicable as positive electrode for a rechargeable solid state lithium ion battery. The cathode material improves the battery performance, such as the rate capability.
Secondary lithium ion batteries are currently the technology of choice, especially for portable applications like mobile devices and notebooks, due to their advantage of high voltage, high volumetric and gravimetric energy density, and long cycle life. However, the high cell voltage of a Li ion battery poses the problem of electrolyte instability since, at high voltages, an aqueous electrolyte will decompose. The alternative organic solvents containing supporting salts are flammable and cause safety issues in the Li ion battery. Especially, with the gradually increasing demand of large-size Li ion batteries, large amounts of combustible electrolyte are required and applied in the devices. This leads to a serious safety issue, such as high potentiality of leaking, overheating and burning. Thus, a solid state electrolyte is expected to be a solution to this problem, due to its non-flammability.
Starting from the middle of the 20th century, the demand for higher safety has boosted the development of solid state lithium ion batteries. The use of a solid electrolyte cannot only solve the safety concerns by its non-flammability, but also provide the possibility to achieve a higher energy density and excellent cyclability. Solid electrolyte has the property of “single-ion” conduction. Typical liquid electrolytes are binary conductors having anodic and cathodic ionic conductivity which causes unwanted effects, for example electrolyte salt depletion. Thus single ion electrolytes in principle can show a superior sustainable power. Additionally a solid/solid interface can be less reactive compared to a liquid electrolyte/cathode interface. This reduces side reactions happening between liquid electrolyte and electrode materials, further preventing the decomposition of electrolyte and finally improving battery life. The advantage of less side reactions also allows the application of solid state lithium ion batteries at high voltage. In “Journal of the Electrochemical Society, 149 (11), A1442-A1447, 2002”, the use of Lithium phosphorous oxy-nitride “LiPON” electrolytes (with the composition LixPOyNz, where x=2y+3z−5) assists cathode material of LiCoO2 (further referred to as LCO) to achieve a high capacity of 170 mAh/g at 4.4V. In “Electrochemical and Solid-State Letters, 11 (6), A97-A100, 2008”, a high capacity at elevated voltage is achieved with the assistance of 0.01Li3PO4-0.63Li2S-0.36SiS2 sulfide glass electrolytes. The combination of high voltage cathode material and solid electrolyte is expected to be useful in applications requiring a high energy density. Besides, the use of solid electrolytes can simplify the battery structure and reduce the number of safeguards in the battery design, which lower the energy density when using liquid electrolyte.
Despite its promising characteristics there are still big limitations to a practical application, the biggest disadvantage of solid-state lithium ion batteries being the low power density. This is considered to be caused by the low ionic conductivity of solid electrolytes, as well as a large charge transfer resistance between solid electrolytes and cathode materials. Currently, many published studies have attempted to improve the bulk conductive properties of various solid electrolytes, like glass electrolyte and polymer electrolyte. Polymer electrolytes are flexible and easy for making close contact between electrolyte and electrodes, but their ionic conductivity and transport number of lithium ions are not satisfying. Glass electrolytes have a relatively high ionic conductivity. Li2S—P2S5 glass ceramic was proposed as one of the most promising solid electrolyte systems in “Advanced Material, 17, 918, 2015”. This literature indicated 70Li2S-30P2S5 showing a good stability against both electrodes at high voltage, and having a high conductivity of 3.2×10−3 S/cm, which is greater by three orders of magnitude than the conventional Lipon thin film solid electrolyte. However, a drawback has still remained hidden, which is that the power densities of solid-state lithium batteries are not comparable with that of organic-solvent liquid electrolytes, in spite of the high ionic conductivities. To further solve this issue, the charge-transfer resistance at the electrode/electrolyte interface has to be considered, especially for the cathode/electrolyte interface. This parameter is essential to fabricate high power density batteries, because the rate of charge transfer at the electrode/electrolyte interface directly relates to the battery performance. A few investigations have focused on this topic.
Most of the prior art tried to reduce the charge-transfer resistance at the positive electrode/solid electrolyte interface by providing a buffer or contacting layer on top of the positive electrode. In WO2015-050031 A1, a layer of Li ion conductive oxide is coated on the cathode, particularly LiNbO3, LiBO2, etc. In WO2015-045921 A1, an active substance layer is applied on the surface of the positive electrode, which comprises positive active material, solid electrolyte and a conduction auxiliary agent. U.S. Pat. No. 7,993,782 B2 disclosed a layer comprising Li ion conductive titanium oxide interposed between cathode material and sulfide electrolyte, to avoid the formation of a high resistance layer at a potential of 4V or more. The above prior art presents the advantage of reducing the interfacial charge transfer resistance, which may lead to improved power performance in solid state batteries, but this has not been proved in their publications.
The charge-transfer resistance at the cathode/electrolyte interface can be decreased not only by adding a conductive surface layer, but also by increasing the contacting area between cathode and electrolyte, which can be understood as requiring a high specific surface area for the cathode particles and a close contact between cathode particle and electrolyte. Normally, a high specific surface area is achieved by providing small particles or porous big particles. But in the case of solid state lithium ion batteries, a small particle size of cathode material would result in a low packing density, which further cuts down the energy density; combined with the consumption of large amounts of electrolyte, which may raise the cost. The use of big porous particles makes it difficult to avoid inner pores, which may result in a poor contact with the electrolyte. Thus, it is necessary to provide a new model of particle morphology of cathode material to increase the specific surface area and finally satisfy the power demand of solid state lithium ion batteries. This is also the target of this invention.
More specifically, this invention aims to develop a cathode material having a large bulk particle size and high BET, which provides excellent power performance when applied in solid state lithium ion batteries.